Optical Power Variation Research Articles

Open-Loop Operation of MEMS-Based<tex>$1,times N$</tex>Wavelength-Selective Switch With Long-Term Stability and Repeatability

We report on a microelectromechanical-systems (MEMS)-based 1/spl times/N wavelength-selective switch (WSS) with excellent open-loop stability and repeatability. The optical power fluctuation of the WSS is </spl plusmn/0.0035 dB over a 3.5-h period. The variation of optical power during 30 ON-OFF switching periods is </spl plusmn/0.0026 dB. Experiments were also performed on the analog MEMS mirror. The stability and repeatability are /spl plusmn/0.00085/spl deg/ and /spl plusmn/0.0013/spl deg/, respectively.

A Wannier-Stark waveguide electroabsorption modulator as a low-power switch and its application as NAND, NOR or EXOR all-optical logic gates

A resistor-biased self-electro-optic-effect device in waveguide geometry has been realized with an electroabsorption modulator containing an InGaAs/InP superlattice as the waveguide core. Room-temperature all-optical switching has been experimentally observed over a wide wavelength range between 1.51 and 1.58 µm. Simulation of the device response to time-varying digital optical input signals shows that it is possible to obtain optical logic operation using these devices. Boolean functions such as NAND and NOR can be realized at 1.55 µm and EXOR operation at 1.51 µm at fairly low optical power levels. The functionality is however rather sensitive to optical bias power variations.

Integrated tunable fiber gratings for dispersion management in high-bit rate systems

Dispersion management is becoming paramount in high-speed wavelength-division-multiplexed lightwave systems, that operate at per-channel rates of 40 Gb/s and higher. The dispersion tolerances, in these systems, are small enough that sources of dispersion variation, that are negligible in slower systems, become critically important to network performance. At these high-bit rates, active dispersion compensation modules may be required to respond dynamically to changes occurring in the network, such as variations in the per-channel power, reconfigurations of the channel's path that are caused by add-drop operations, and environmental changes, such as changes in ambient temperature. We present a comprehensive discussion of an emerging tunable dispersion compensating device, based on thermally actuated fiber gratings. These per-channel devices rely on a distributed on-fiber thin film heater, deposited onto the outer surface of a fiber Bragg grating. Current flowing through the thin film generates resistive heating at rates that are governed by the thickness profile of the metal film. A chirp in the grating is obtained by using a thin-film, whose thickness varies with position along the length of the grating in a prescribed manner; the chirp rate is adjusted by varying the applied current. The paper reviews some of the basic characteristics of these devices and their implementation, in a range of different applications, including the mitigation of power penalties associated with optical power variations. We present detailed analysis of the impact of group-delay ripple and polarization-mode dispersion on systems performance, and present results from systems experiments, that demonstrate the performance of these devices at bit rates of 10, 20, 40 and 160 Gb/s. We also discuss advantages and disadvantages of this technology, and compare to other devices.

Variable focal lens controlled by an external voltage: An application of electrowetting

We use electrocapillarity in order to change the contact angle of a transparent drop, thus realizing a lens of variable focal length (B. Berge, J. Peseux, Patent deposited in Grenoble France, October 8th 1997, numero d'enregistrement national 97 12781). The key point is the application of gradients of wettability, which control the shape of the drop edge, in our case a centered circle of variable radius. The quality and reversibility of the lens are surprisingly good. The optical power variation can be 5 to 10 times the one of the human eye, for a comparable diameter, with a typical response time of 0.03 s and a dissipated power of a few mW.

Absolute distance measurement with an optical feedback interferometer

An important use of the self-mixing effect inside a frequency-modulated single-mode laser diode is in laser velocimetry and range-finding applications. The optical beam reflected by a target and injected into the laser diode cavity modulated by a reshaped current is mixed with the light inside the cavity, causing variations of the optical output power. A theoretical analysis of this effect is proposed, based on the determination of the beat frequencies of the optical power variations, to improve the accuracy of laser distance measurement. A resolution of ?1.5 mm from 50 cm to 2 m is obtained when thermal effects are taken into account.

Backscatter-modulated laser diode for low-frequency small-amplitude vibration measurement

Feedback phenomenon in laser diodes is applied to low-frequency vibration measurement in the micrometer range on poorly reflecting targets. The laser diode is frequency modulated. Its beam is focused on the vibrating target, and backscattered light is fed back into the laser. The optical laser diode power variation is processed to measure the frequency and amplitude of low-frequency vibrations of amplitude, ~100 nm to 10 mum. Experimental results for sinusoidal and triangular vibrations are compared with theory.

Photoelectrical properties of semiconductor tips in scanning tunneling microscopy.

We describe a model as well as experiments on the electrical properties of a photoexcited tunnel junction between a metal and a semiconductor material, as is established in a scanning tunneling microscope. The model treats the case in which carrier transport is mediated by capture and relaxation in the semiconductor surface states. In the semiconductor, majority carrier transport is determined by thermionic emission over the Schottky barrier and subsequent surface recombination. By optical excitation an additional minority carrier current is generated. The voltage that develops on the semiconductor surface is determined by the balance between majority and minority carrier current in the semiconductor, and the current across the tunnel barrier. We present model calculations of the (nonplanar) band-bending profile in the semiconductor, which indicate that the subsurface electric field operates as an electrical lens that can focus or defocus the current. Measurements were performed with moderately doped GaAs tips or samples prepared by cleavage. Continuous as well as modulated photoexcitation was used. Relationships are determined between tunnel current, applied voltage, incident optical power, and tip-sample distance. The experimental results are well described by the model that includes carrier capture in the semiconductor surface states. It is shown that the sensitivity of the tunnel current to small variations in optical power is determined by the ratio of the tunnel barrier conductance to the Schottky barrier conductance. The implications for near-field optical imaging and spin-polarized tunneling with semiconductor tips are discussed. \textcopyright{} 1996 The American Physical Society.

Photoamperic probes in scanning tunneling microscopy

The charge generated at the apex of a semiconductor tip upon laser irradiation is utilized in a scanning tunneling microscope. We show such arrangements can produce photoinduced tunnel currents of several hundred picoamperes, sufficient for stable STM operation and sensitive enough to detect nanowatt variations in the incident optical power.

200-Mb/s/ch 100-m optical subsystem interconnections using 8-channel 1.3-μm laser diode arrays and single-mode fiber arrays

Synchronous parallel optical-fiber transmission is an attractive method for providing increased interconnection throughput and higher density in advanced information systems. Skew suppression (reducing transmission delay time variation), error-free fully DC-coupled data transmission capability, compactness, and low power consumption of modules are important requirements. In order to meet these requirements, we developed optical subsystem interconnections using long-wavelength laser diode arrays and single-mode fiber arrays. The major design criteria are discussed, especially as they are related to skew due to laser diode turn-on delay and receiver input optical power variation. The use of low-threshold-current laser diode arrays is an important design requirement. Based on the design criteria discussed in this work, we demonstrated and channel 200-Mb/s/ch 100-m transmission using compact (0.18 cc/ch/module), low power (total 280 mW/ch), fully integrated transmitter and receiver modules with an ECL (emitter coupled logic) interface. These modules include our new laser diode arrays with low threshold current of 3.2 mA. Performance results showed that these modules are very effective as interconnections between synchronously operating subsystems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Remote detection of methane with a 166-μm diode laser

High-sensitivity real-time remote detection of methane in air with a 1.66-microm distributed-feedback diode laser operating at room temperature is demonstrated by laboratory simulations. The laser current was modulated at a high frequency of ~5 MHz, and the laser-center frequency was locked onto a methane-absorption line. The laser light directed toward the probed region was received after one-way transmission or further reflection from a topographic target. The methane absorption was detected by the second-harmonic component in the optical-power variation. The minimum-detectable concentration-path-length product in the transmission scheme was 0.3 part in 10(6) m for a signal averaging time of 1.3 s. In the reflection scheme, the amount of methane could be measured from the ratio of the fundamental and second-harmonic signal intensities independently of the received power.

The influence of the carrier-dependent shifts of the longitudinal modes and the gain profile on the emission characteristics of a pulsed Al x Ga1?x As double heterostructure laser

The wavelength of the longitudinal modes of a stripe geometry DH laser excited by current pulses is shifted according to the variation of the optical power. The magnitude of the spectral shift and the different form of these patterns are due to the mutual influence of individual modes. The time-varying carrier concentration gives rise to a time-dependent (effective) refractive index which alters the resonance condition for the modes. The spectral broadening of the modes, spanning nearly one mode separation, is due to the spectral shift of the modes which in turn depend on the variation of the emitted power. The longitudinal modes are sequentially excited because the gain profile changes its wavelength position with varying carrier concentration.

Planar star coupler for multimode fibers

T-shaped network configurations cannot be used for optical data bus systems intended for passive communication between more than about ten terminals because of balanced power distribution and the severe requirements that would have to be met with respect to the dynamic range of the detectors. Star network configurations, however, allow passive communication between several hundreds of terminals using state-of-the-art fibers, transmitters, and detectors. The planar mixers used in such star couplers are investigated from both theoretical and experimental aspects with regard to their influence on the uniformity of optical power distribution and insertion losses. Measurements on a star coupler with twelve fibers of 200-microm core diam and 15-microm cladding thickness showed an optical power variation of ~+/-12% along the entire width of the mixer accompanied by an insertion loss of ~3dB.

Holographic Optical Elements

This article is a survey of the characteristics, technology and applications of Holographic Optical Elements (HOEs), considered as general elements in an optical system. HOEs function by diffraction of light from a generalized grating structure with nonuniform groove spacing. HOEs provide a system of thin film optics. They are capable of unique system functions and configurations, show a rapid variation of optical power and image characteristics with wavelength, have relatively large amounts of astigmatism and coma, and require special consideration of optical efficiency during system design. Comparison of the aberrations of specific, f/3.3 elements shows that the on-axis HOE and the conventional glass lens element have similar aberration levels, while the off-axis HOE has four times as much astigmatism and twice as much coma. We show that these grating aberrations, which appear for conjugate points that are different from the HOE construction points, are proportional to the average surface grating spatial frequency of an off-axis HOE. HOE technology is similar to conventional optics technology, but is less developed. The relative complexity of optical systems with HOEs, and the lack of a suitable aberration theory, produce a relince on computer-based raytracing for system design and development. We give the basic raytracing equations and discuss the special requirements for hologram recording appara-tus and materials. In applications, HOEs will usually provide unique capabilities rather than replace conventional elements, and will usually operate over narrow spectral bandwidths. The use of HOEs in several types of laser optical systems, and in visual displays, appears to be not only advantageous, but also technically and economically feasible.

Optically active co-ordination compounds. Part VII. Tris-complexes of (+)-hydroxymethylenecamphor

The tris-complexes of (+)-hydroxymethylenecamphorate with chromium(III), cobalt(III), and rhodium(III) have been investigated in detail. The geometrical-optical isomers obtained pure are characterised by studies of electronic, and proton resonance spectra, and Cotton effect. The isomers obtained from cobalt(III) are D-1,2,3-(–)[Co(+hmc)3] and D-1,2,6-(–)-[Co(+hmc)3], while the sole isolable product containing chromium(III) is D-1,2,6-(+)-[Cr(+hmc)3], though an unstable L-isomer may also exist. However, with rhodium(III), the isomers D-1,2,6-(–)-[Hh(+hmc)3] and an L-isomer, probably L-1,2,6(+)-[Rh(+hmc)3] were isolated. A distinction is made between kinetic and thermodynamic control of stereoselectivity in complexes. The absolute configurations of all the isolated isomers of [M(+hmc)3] are given. The variation of optical rotatory power of (–)-[Co(+hmc)3] as a function of solvent is presented, and correlated with properties of the solvent and of the complex, in particular its absorption spectrum.